Which of the following statements is CORRECT?

The assertion that all of the statements provided are correct highlights a comprehensive understanding of pressure measurements and terminology used in various contexts, especially in fields such as hydraulics and fluid mechanics.

The first statement regarding positive pressures being measured in pounds per square inch is valid, as this is a common unit used for gauging the pressure of fluids in systems. Pounds per square inch (psi) quantifies the amount of force exerted over a specific area, making it a standard measurement for positive pressures.

The second statement about negative pressures, specifically measuring in inches of mercury, is also accurate. Inches of mercury (inHg) is a traditional unit of pressure often used in barometric readings and vacuum measurements, indicating the height of a column of mercury that a given pressure can support.

The third statement clarifies that any pressure less than atmospheric pressure is considered a vacuum. This is a fundamental concept in physics; a vacuum is defined as a space devoid of matter, which significantly reduces the pressure compared to that of the surrounding atmosphere.

Since all three statements accurately describe essential principles of pressure measurement and terminology, the combination of these facts leads to the conclusion that all of the above statements are correct.